Honeycomb ceramics filter

ABSTRACT

There is disclosed a honeycomb ceramics filter, wherein an average pore diameter X (μm) and partition wall thickness W (μm) of the filter satisfy a relation of 10≧W/X. According to this filter, while a predetermined trapping efficiency is maintained, a pressure loss can be prevented from increasing to be not less than a predetermined pressure loss even with use for a long period.

TECHNICAL FIELD

[0001] The present invention relates to a honeycomb ceramics filterwhich has a honeycomb shape. The present invention further particularlyrelates to a honeycomb ceramics filter in which trapping efficiency ofparticulates is sacrificed to some extent, but an ash content generatedby oil combustion of a diesel engine is extracted from pores so as toprevent a pressure loss from increasing to be not less than apredetermined loss even with a long-period use.

BACKGROUND ART

[0002] In recent years, a porous honeycomb ceramics filter has been usedas an apparatus for removing particulates in exhaust gas, including astructure in which a plurality of through holes opened in end surfaceson exhaust gas inflow and outflow sides are alternately closed in theopposite end surfaces. The exhaust gas which has flown in from the endsurface on the exhaust gas inflow side is forcibly passed throughpartition walls (including a plurality of fine pores) among therespective through holes to trap/remove the particulates in the exhaustgas.

[0003] For the ceramics filter, a pore diameter of the fine pore formedin the partition wall between the respective through holes is determinedby a relation with a particulate diameter of the particulate in theexhaust gas, and capabilities such as trapping efficiency and pressureloss differ with a degree of the pore diameter. In view of this,development has heretofore been advanced so as to obtain the ceramicsfilter having a high trapping efficiency of particulates (such as soot).

[0004] However, for the filter having the raised trapping efficiency,since the pore diameter of the filter is small, there is a problem thatan ash content generated by oil combustion of a diesel engine isaccumulated in the pores of the filter and the pressure loss is raisedto be not less than a predetermined loss by the use for a long period.For the filter having a small pore diameter, the pressure loss is highin sampling the particulates (such as the soot), and there has been ademand for reduction of the loss.

[0005] Additionally, in recent years, by improvement of the dieselengine, the particulates such as the soot generated from the engine havebeen reduced by a considerable amount than before. Then, as a result ofthe present inventor's intensive studies, in order to clear a regulatedvalue of a future exhaust gas standard, it is not necessary to achievean extremely high trapping efficiency. Conversely, it has been foundthat in order to prevent a rise of the pressure loss caused by theaccumulation of the ash content into the pores, it is important toincrease the pore diameter to such an extent that the exhaust gas canpass through the partition wall, and the present invention has beenattained.

[0006] Therefore, an object of the present invention is to provide ahoneycomb ceramics filter in which a predetermined trapping efficiencyis maintained, an ash content generated by oil combustion of a dieselengine is extracted from pores, and pressure loss is accordinglyprevented from increasing to be not less than a predetermined loss evenwith long-period use.

DISCLOSURE OF THE INVENTION

[0007] That is, according to the present invention, there is provided ahoneycomb ceramics filter, characterized in that an average porediameter X (μm) and partition wall thickness W (μm) of the filtersatisfy the following relation:

10≧W/X.

[0008] In the ceramics filter of the present invention, the average porediameter X (μm) and partition wall thickness W (μm) of the filterpreferably satisfy a relation of 7≧W/X≧3. The average pore diameter X(μm) and partition wall thickness W (μm) of the filter more preferablysatisfy a relation of 5≧W/X≧3.

[0009] Moreover, in the ceramics filter of the present invention, aporosity is preferably 55 to 75%, and a ceramic preferably containscordierite and/or silicon carbide as a major component. Furthermore, inthe ceramics filter of the present invention, a thermal expansioncoefficient at 40 to 800° C. is preferably 1.0×10⁻⁶/° C. or less. Thefilter of the present invention preferably has a honeycomb shape whosepartition wall thickness W is 350 μm or less and whose cell density is250 cells/in² or more.

BEST MODE FOR CARRYING OUT THE INVENTION

[0010] Embodiments of the present invention will concretely be describedhereinafter, but the present invention is not limited to theseembodiments.

[0011] In the present invention, a pore diameter and partition wallthickness are controlled so that an average pore diameter X (μm) andpartition wall thickness W (μm) of the filter satisfy a relation of10≧W/X to constitute a honeycomb ceramics filter.

[0012] As described above, the present inventor has found that it isimportant to increase a pore diameter to such an extent that exhaust gascan pass through partition walls in order to prevent a rise of pressureloss caused by accumulation of an ash content into pores. Concretely,the present inventor has intensively studied a relation between the porediameter and the partition wall thickness. When the pore diameter isincreased to be not less than a predetermined diameter with respect tothe partition wall thickness of a honeycomb structure, trappingefficiency of particulates such as soot drops to some extent, but thepressure loss at the time of sampling the particulates can be reduced.Additionally, the ash content accumulated in the pores formed insidepartition walls can be extracted from the pores. As a result, it hasbeen found that it is possible to obtain a honeycomb ceramics filterwhich can stably function for a long period without increasing thepressure loss to be not less than the predetermined loss even with thelong-period use.

[0013] In the present invention, when a relation W/X between the averagepore diameter X (μm) and partition wall thickness W (μm) of the filteris larger than 10, the exhaust gas does not easily pass through thepartition walls, and the ash content accumulated in the pores cannot beextracted from the pores.

[0014] In the present invention, the relation between the average porediameter X (μm) and the partition wall thickness W (μm) is preferably7≧W/X≧3 in order to achieve the above-described pores, and morepreferably 5≧W/X≧3.

[0015] Moreover, in the honeycomb ceramics filter of the presentinvention, in order to achieve the above-described object of the presentinvention, the average pore diameter X is usually 20 to 70 μm, morepreferably 30 to 70 μm. The partition wall thickness W is preferably 350μm or less, and more preferably in a range of 200 to 300 μm.

[0016] Major components of the ceramics filter of the present inventionare not especially limited, and any type of ceramic can be used, butcordierite and/or silicon carbide are preferably the major components.Cordierite may be oriented, non-oriented, α-crystalline, orβ-crystalline. Silicon carbide may be either α-crystalline orβ-crystalline.

[0017] Moreover, other components may also be contained such as mullite,zirconium, aluminum titanate, clay bond silicon carbide, zirconia,spinel, indialite, sapphirine, corundum, and titania.

[0018] The honeycomb filter of the present invention has a porosity ofpreferably 55 to 75%, more preferably 60 to 70% in terms of thereduction of the pressure loss and the trapping efficiency. In terms ofenhancement of thermal shock resistance at the time of the use at a hightemperature, a thermal expansion coefficient at 40 to 800° C. ispreferably 1.0×10⁻⁶/° C. or less, more preferably 0.8×10⁻⁶/° C. or less.

[0019] Moreover, the ceramics filter of the present invention is usuallyof a honeycomb type including a structure in which a plurality ofthrough holes opened in end surfaces on exhaust gas inflow and outflowsides are alternately closed in the opposite end surfaces. However, theshape of the honeycomb filter is not especially limited, and any ofshapes may be used such as a column whose end surface has a perfectcircular or elliptic shape, a prism whose end surface has a triangular,quadrangular, or otherwise polygonal shape, and a column or prism whoseside surface has a curved V-shape. Moreover, the shape of the throughhole is not especially limited, and the section may have any of theshapes such as the polygonal shapes including the quadrangular andoctagonal shapes, the perfect circular shape, and the elliptic shape.The filter has a cell density in a range of preferably 250 cells/in² ormore, more preferably 300 to 400 cells/in² in terms of the samplingcapability of the exhaust gas.

[0020] The honeycomb ceramics filter of the present invention canbe-manufactured by the following methods.

[0021] First, when a cordierite material is used as a starting materialof the filter, and when the cordierite material is blended withcomponents so as to obtain a theoretical composition of a cordieritecrystal, it is necessary to blend silica (SiO₂) source components,magnesia (MgO) source components such as kaolin and talc, alumina(Al₂O₃) source components such as aluminum oxide and aluminum hydroxide,and the like.

[0022] For the alumina (Al₂O₃) source components, the materialcontaining either one or both of aluminum oxide and aluminum hydroxideis preferable because of a small amount of impurities. Above all, it ispreferable to contain aluminum hydroxide.

[0023] Moreover, for the alumina (Al₂O₃) source components, thecordierite material preferably contains 15 to 45% by mass of aluminumhydroxide, or 0 to 20% by mass of aluminum oxide.

[0024] Examples of the magnesia (MgO) source components include talc andmagnesite. Above all, talc is preferable. The cordierite materialpreferably contains 37 to 40% by mass of talc, and a particle diameterof talc is preferably 5 to 40 μm, more preferably 10 to 30 μm in orderto reduce the thermal expansion coefficient.

[0025] Moreover, the magnesia (MgO) source components such as talc foruse in the present invention may contain Fe₂O₃, CaO, Na₂O, K₂O, and thelike which are impurities. Additionally, the content of Fe₂O₃ ispreferably 0.1 to 2.5% by mass in the magnesia (MgO) source component.With the content in this range, the thermal expansion coefficient can bereduced, and a high porosity can also be obtained.

[0026] Furthermore, the contents of CaO, Na₂O, K₂O are preferably set to0.35% by mass or less in total in the magnesia (MgO) source component soas to lower the thermal expansion coefficient.

[0027] It is to be noted that in the present invention, silicon carbidecan be the major component as the starting material of the filter. Thecase in which silicon carbide is the major component includes both thecases in which silicon carbide (SiC) is the major component and in whichsilicon carbide (SiC) and metal silicon (Si) are the major components.

[0028] When the filter of the present invention is manufactured, thestart material containing the cordierite material and/or silicon carbideas the major component may be blended with various additives ifnecessary. Examples of the additives include a foamed resin, a binder, adispersant for promoting dispersion into a medium solution, and a holemaking material for forming pores.

[0029] Examples of the foamed resin include acrylic microcapsule, andthe like. Examples of the binder include hydroxypropyl methyl cellulose,methyl cellulose, hydroxyl ethyl cellulose, carboxyl methyl cellulose,polyvinyl alcohol, and the like. Examples of the dispersant includeethylene glycol, dextrin, fatty acid soap, polyalcohol, and the like.Examples of the hole making agent include graphite, flour, starch,phenol resin, polymethyl methacrylate, polyethylene, polyethyleneterephthalate, and the like.

[0030] These additives can be used alone or as a combination of two ormore thereof in accordance with purposes.

[0031] In the present invention, the above-described starting materialscan be used to manufacture the honeycomb ceramics filter in thefollowing manufacturing steps.

[0032] First, 3 to 5 parts by weight of the binder, 3 to 40 parts byweight of the hole making agent, 0.5 to 2 parts by weight of thedispersant, and 10 to 40 parts by weight of water are cast into 100parts by weight of the above- described starting material, kneaded, andplasticized.

[0033] Subsequently, the plastic material can be molded by an extrusionmolding method, an injection molding method, a press molding method, amethod of molding a ceramic material into a columnar shape andthereafter forming through holes, and the like. Above all, the extrusionmolding method is preferably carried out in that continuous molding iseasy and, for example, a cordierite crystal can be oriented to obtain alow thermal expansion property.

[0034] Next, the molded material can be dried by hot-air drying,microwave drying, dielectric drying, reduced-pressure drying, vacuumdrying, freeze drying, and the like. Above all, a drying step in whichthe hot-air drying is combined with the microwave drying or thedielectric drying is preferably carried out in that the whole materialcan quickly and uniformly be dried.

[0035] Finally, to calcine the dried/molded material, depending on thesize of the dried/molded material, usually with the cordierite material,the material is preferably calcined under the atmosphere at temperatureof 1410 to 1440° C. for three to seven hours. With the materialcontaining silicon carbide which is the major component, the material iscalcined under a non-oxidizing atmosphere such as N₂ and Ar in order toprevent-oxidation of SiC. When SiC is bonded by silicon nitride, thecalcining temperature is a temperature at which a silicon nitride powderis softened, and the calcining is preferably carried out at atemperature of 1550 to 2000° C. When SiC particles are bonded to oneanother by a recrystallization method, it is necessary to calcine thematerial at a temperature of at least 1800° C. or more. Furthermore,when SiC and Si are major components, the calcining is preferablycarried out under the non-oxidizing atmosphere such as N₂ and Ar at atemperature of 1400 to 1800° C. It is to be noted that the drying andcalcining steps may also continuously be carried out.

[0036] The present invention will more concretely be described by meansof examples hereinafter, but the present invention is not limited tothese examples.

[0037] 1. Evaluation Method

[0038] The honeycomb ceramics filters obtained in Examples 1 to 13 andComparative Examples 1 to 5 described later were evaluated by thefollowing methods.

[0039] (1) Average Pore Diameter

[0040] The average pore diameter was measured by a mercury press-inporosimeter manufactured by Micromeritics Co.

[0041] (2) Porosity

[0042] A true specific gravity of cordierite was set to 2.52 g/cm³, andthe porosity was calculated from a total pore volume. The true specificgravity of SiC was set to 3.05 g/cm³.

[0043] (3) Trapping efficiency

[0044] Exhaust gas from which soot was generated by a soot generator wasallowed to flow into a filter having a size: φ144 mm×152 mm (length) andhaving an average pore diameter X, partition wall thickness W, porosity,cell density, and thermal expansion coefficient shown in Table 2 for twominutes. The soot included in the exhaust gas passed through the filterwas trapd with filter paper, and a weight (W₁) of the soot was measured.At the same time, the exhaust gas from which the soot was generated wastrapd with the filter paper without being passed through the filter, anda weight (W₂) of the soot was measured. Subsequently, the obtainedweights (W₁) and (W₂) were assigned to the following equation to obtainthe trapping efficiency:

(W ₂ −W ₁)/(W ₂)×100.

[0045] (4) Soot Sampling Pressure Loss Evaluation

[0046] The honeycomb ceramics filter having the size: φ144 mm×152 mm(length) was used, front and back of the honeycomb filter were pressedwith a ring having an inner diameter of φ130 mm, and measurement wascarried out substantially in the inner diameter of φ130 mm. The soot wasgenerated by the soot generator, and 10 g of soot was trapd by thehoneycomb filter. In this state, 2.27 Nm³/min of air was passed, and apressure difference before/after the filter was measured.

EXAMPLES 1 TO 12

[0047] Main materials and hole making materials were mixed to preparevarious cordierite materials with average particle diameters and blendratios shown in Table 1.

[0048] Subsequently, with respect to 100 g of each of these variouscordierite materials, 4 g of hydroxypropyl methyl cellulose, 0.5 g oflauric acid potash soap, and 30 g of water were cast, kneaded, andplasticized. This plastic material was formed into a cylindrical moundof earth with a vacuum earth kneader, cast into an extrusion moldingmachine, and molded in a honeycomb shape.

[0049] Subsequently, various obtained molded materials weredielectrically dried, and thereafter absolutely dried by the hot-airdrying, and opposite end surfaces were cut into predetermineddimensions.

[0050] Subsequently, the through holes in the honeycomb-shaped driedmaterial were alternately closed in the opposite end surfaces in whichthe through holes were opened with a flurry formed of the cordieritematerial having the similar composition.

[0051] Finally, after the calcining at 1420° C. for four hours,according to Examples 1 to 12 and Comparative Examples 1 to 5, thehoneycomb ceramics filter having the size: φ144 mm×152 mm (length) wasobtained.

EXAMPLE 13

[0052] The main materials (SiC and Si) and hole making materials shownin Table 1 were used to carry out the drying and hole closing in thesame method as that of Examples 1 to 12. The materials were calcinedbelow 400° C. in an ambient-pressure oxidizing atmosphere, at 400° C. ormore in an ambient-pressure argon atmosphere, and at a maximumtemperature of 1450° C. for one hour to obtain the honeycomb ceramicsfilter having the size: φ144 mm×152 mm (length).

[0053] Evaluation results of Examples 1 to 13 and Comparative Examples 1to 5 are shown altogether in Table 2. TABLE 1 Main material Hole makingmaterial Molten Aluminum Aluminum Foamed Talc Kaolin Quartz silica oxidehydroxide Graphite resin No. (% by mass) (% by mass) (% by mass) (% bymass) (% by mass) (% by mass) (% by mass) (% by mass) Example 1 40 (15μm) 19 (10 μm) 12 (39 μm) 0 14  (7 μm) 15   (2 μm) 15 (51 μm) 2 (52 μm)Example 2 40 (15 μm) 19 (10 μm) 12 (39 μm) 0 14  (7 μm) 15   (2 μm) 15(51 μm) 2 (52 μm) Example 3 40 (15 μm) 19 (10 μm) 12 (39 μm) 0 14  (7μm) 15   (2 μm) 15 (51 μm) 2 (52 μm) Example 4 40 (15 μm) 0 21 (10 μm) 016  (7 μm) 23   (3 μm) 20 (29 μm) 4 (35 μm) Example 5 40 (15 μm) 0 21(10 μm) 0 16  (7 μm) 23   (3 μm) 20 (29 μm) 4 (35 μm) Example 6 40 (15μm) 0 21 (10 μm) 0 16  (7 μm) 23   (3 μm) 20 (29 μm) 4 (35 μm) Example 740 (15 μm) 0 21 (10 μm) 0 16  (7 μm) 23   (3 μm) 20 (29 μm) 4 (35 μm)Example 8 40  (7 μm) 0 0 21 (78 μm) 16  (5 μm) 23 (1.5 μm) 20 (29 μm) 4(64 μm) Example 9 40 (20 μm) 0 21 (89 μm) 0 16 (10 μm) 23 (1.5 μm) 0 3(64 μm) Example 10 40 (20 μm) 0 21 (89 μm) 0 16 (10 μm) 23 (1.5 μm) 0 3(64 μm) Example 11 40 (20 μm) 0 0 21 (54 μm) 16 (10 μm) 23   (2 μm) 0 2(52 μm) Example 12 40 (15 μm) 19 (10 μm) 0 12 (32 μm) 14  (7 μm) 15   (2μm) 20 (29 μm) 4 (35 μm) Example 13 SiC (35 μm): 80% by mass, Si (6 μm):20% by mass 0 3 (52 μm) Comparative 40 (15 μm) 0 21 (10 μm) 0 16  (7 μm)23   (3 μm) 20 (29 μm) 4 (35 μm) Example 1 Comparative 40 (15 μm) 19 (10μm) 12 (10 μm) 0 14  (7 μm) 15   (2 μm) 15 (51 μm) 3 (52 μm) Example 2Comparative 40 (15 μm) 19 (10 μm) 12 (10 μm) 0 14  (7 μm) 15   (2 μm) 15(51 μm) 3 (52 μm) Example 3 Comparative 40 (15 μm) 19 (10 μm) 12 (10 μm)0 14  (7 μm) 15   (2 μm) 15 (51 μm) 3 (52 μm) Example 4 Comparative 40(15 μm) 19 (10 μm) 12 (10 μm) 0 14  (7 μm) 15   (2 μm) 15 (51 μm) 3 (52μm) Example 5

[0054] TABLE 2 Partition Average Thermal wall Cell pore expansion Sootsampling Trapping thickness W density Porosity diameter X coefficientpressure loss efficiency No. (μm) (/in²) (%) (μm) W/X (x10⁻⁶/° C.) (KPa)(%) Example 1 300 300 59 32 9.4 0.6 9.4 86 Example 2 250 300 59 32 7.80.6 8.6 71 Example 3 200 300 59 32 6.3 0.6 7.3 59 Example 4 200 350 7024 8.3 0.7 5.2 79 Example 5 150 350 70 24 6.3 0.7 4.9 58 Example 6 150400 70 24 6.3 0.7 4.2 52 Example 7 100 400 70 24 4.2 0.7 3.5 45 Example8 300 300 75 45 6.7 1.0 7.6 63 Example 9 300 300 61 61 4.9 1.0 9.3 48Example 10 350 300 61 61 5.7 1.0 10.3 50 Example 11 200 300 55 45 4.40.5 7.9 45 Example 12 100 400 68 32 3.1 0.8 3.7 42 Example 13 300 250 5831 9.7 4.1 7.3 89 Comparative 250 300 70 24 10.4 0.7 7.1 91 Example 1Comparative 200 300 68 15 13.3 0.8 6.8 94 Example 2 Comparative 300 30068 15 20.0 0.8 8.6 96 Example 3 Comparative 400 300 68 15 26.7 0.6 10.397 Example 4 Comparative 300 200 68 15 20.0 0.6 10.7 95 Example 5

Industrial Applicability

[0055] As described above, since a value obtained by dividing apartition wall thickness by an average pore diameter has correlationwith a trapping efficiency, according to the present invention, thevalue obtained by dividing the partition wall thickness by the averagepore diameter can be controlled to freely control the trappingefficiency. Since the trapping efficiency of soot is assumed to havecorrelation with that of an ash content, according to the presentinvention, there can be provided a honeycomb ceramics filter in which apredetermined trapping efficiency is maintained and a pressure loss byaccumulation of the ash content is prevented from increasing to be notless than a predetermined pressure loss even with use for a long period.

1. (Amended) A honeycomb ceramics filter, characterized in that thehoneycomb ceramics filter contains cordierite as a major component, athermal expansion coefficient at 40 to 800° C. is 0.8×10⁻⁶/° C. or less,and an average pore diameter X (μm) and partition wall thickness W (μm)of the filter satisfy the following relation: 10≧W/X.
 2. The honeycombceramics filter according to claim 1, wherein the average pore diameterX (μm) and partition wall thickness W (μm) of the filter satisfy thefollowing relation: 7≧W/X≧3.
 3. The honeycomb ceramics filter accordingto claim 1, wherein the average pore diameter X (μm) and partition wallthickness W (μm) of the filter satisfy the following relation: 5≧W/X≧3.4. The honeycomb ceramics filter according to any one of claims 1 to 3,wherein a porosity is 55 to 75%.
 5. (Deleted)
 6. (Deleted)
 7. (Amended)The honeycomb ceramics filter according to any one of claims 1 to 3,wherein the partition wall thickness W is 350 μm or less.
 8. (Amended)The honeycomb ceramics filter according to any one of claims 1 to 3 and7, wherein a cell density is 250 cells/in² or more.